Back Conversion from Product to Parent: Methyl Triclosan to Triclosan in Plants

Numerous man-made chemicals pass through wastewater treatment plants (WWTPs), where biological and chemical treatments often result in the formation of a wide range of transformation products via reactions such as conjugation and alkylation. Such transformation products may come into contact with plants when treated wastewater and biosolids are used in agricultural production or when plants are used for mitigation purposes (e.g., treatment wetlands). Using the high-volume antimicrobial triclosan as a model compound, we showed that its primary transformation product in the WWTPs, methyl triclosan, was readily converted back to the parent in plants. Upon exposure to environmentally relevant concentrations of methyl triclosan, triclosan was detected in Arabidopsis thaliana cells within 12 h, and the level of triclosan increased over time. The molar fraction of triclosan to methyl triclosan in the cells was estimated to be 0.17 at 144 h. When grown in a nutrient solution containing methyl triclosan, lettuce and carrot seedlings were also capable of transforming methyl triclosan back to triclosan after 4 days, with triclosan levels reaching >10 mu g/g in lettuce tissues and >3 mu g/g in carrot tissues. The back and forth conversions of triclosan in various environmental compartments effectively prolong its environmental persistence and exposure. Future assessment of this and other emerging contaminants should consider such interconversions to obtain a better understanding of their fate and risks.